1. Field of the Invention
The present invention relates to an improvement of an air-fuel ratio sensor which is, for example, installed on an intake or exhaust gas pipe of an internal combustion engine for an automotive vehicle for detecting a ratio of fuel to air in the intake or exhaust gas, and more particularly to an improvement of a waterproof sealing structure applied to a portion where lead wires are fixed by a rubber bush and subjected to a high-temperature environment.
2. Related Art
Conventionally, air-fuel ratio sensors of automotive engines have been located fixedly at various positions in engine exhaust systems. Particularly, when an air-fuel ratio sensor is installed under a floor of a vehicle body, there is a possibility that the air-fuel ratio sensor is exposed to water. Thus, it is necessary to enhance the reliability of the waterproof structure for surely preventing water from entering into the sensor.
In general, any permeation of water into the sensor body causes an undesirable change of the oxygen concentration detected by the sensor. As apparent from the operating principle of the air-fuel ratio sensor, it is already known that such a change possibly results in a malfunction of the air-fuel ratio sensor.
A sealing rubber bush is used at a portion where lead wires are taken out from the sensor body. To improve the waterproof structure of an air-fuel ratio sensor, this kind of sealing rubber bush have -been usually made of a fluorine-contained rubber or silicone rubber having an excellent heat resistivity.
Recent change of automotive vehicle environments, such as enforcement of the latest European regulation on exhaust gas emission (i.e. reduction of harmful components involved in exhaust gas during a high-speed cruising condition), will increase the temperature of exhaust gas as a result of improvement of combustion efficiency in an engine. This will lead to severe deterioration or damage of a rubber bush of an air-fuel ratio sensor installed closely to an engine exhaust pipe in which high-temperature exhaust gas flows. For example, there is a possibility that the rubber bush is subjected to a high-temperature atmosphere of 300.degree. C.
A problem is that a conventional fluorine-contained rubber bush or a silicone rubber bush is speedily decomposed or cracked under such a severe condition of 300.degree. C., damaging the waterproof structure.
Furthermore, there is a tendency that an air-fuel ratio sensor is downsized to improve its installation onto an automotive vehicle. However, such a size reduction of an air-fuel ratio sensor may force the rubber bush to be subjected to a further severe (higher temperature) condition. Thus, the deterioration of the waterproof structure will be expedited.